perf: Optimize hashing, null-free fast path for percentile_cont, median

[neilconway]

Which issue does this PR close?

• Closes Optimize hashing, null-free fast path in percentile_cont, median #23953

Rationale for this change

This PR makes three improvements to non-distinct PercentileContAccumulator and MedianAccumulator, inspired by recent work on percentile_cont(DISTINCT) (#23946):

1. Switch from SipHash to foldhash for internal hash maps
2. Add a null-free fast path to update_batch and retract_batch
3. Raise an error if we attempt to retract an unknown value in retract_batch, rather than silently ignoring it.

Benchmarks:

  percentile_cont no_nulls   window=256    -61.1%  (249.0 -> 96.6 us)
  percentile_cont with_nulls window=256    -59.4%  (232.7 -> 94.5 us)
  percentile_cont no_nulls   window=4096   -50.9%  (756.3 -> 370.4 us)
  percentile_cont with_nulls window=4096   -48.2%  (552.2 -> 286.2 us)
  percentile_cont no_nulls   window=16384  -47.3%  (2.311 -> 1.218 ms)
  percentile_cont with_nulls window=16384  -42.0%  (1.465 -> 0.851 ms)
  median          no_nulls   window=256    -62.8%  (247.0 -> 92.0 us)
  median          with_nulls window=256    -59.8%  (228.8 -> 92.0 us)
  median          no_nulls   window=4096   -40.0%  (411.4 -> 246.6 us)
  median          with_nulls window=4096   -39.4%  (364.4 -> 221.0 us)
  median          no_nulls   window=16384  -31.5%  (1.107 -> 0.759 ms)
  median          with_nulls window=16384  -32.9%  (0.947 -> 0.637 ms)

What changes are included in this PR?

See above.

Are these changes tested?

Yes: existing tests pass, new tests added for the null-free fast path and the improved error handling.

Are there any user-facing changes?

No.

[neilconway]

cc @viirya

[viirya]

LGTM — nice to see the #23946 ideas carried over to the non-distinct accumulators, and the benchmark wins are substantial.

I went through all three changes:

• foldhash on the to_remove map — matches what we did for the distinct path.
• null-free fast path: update_batch uses extend_from_slice(values.values()) when there are no nulls, which is correct since null_count() == 0 guarantees every slot in the values buffer is a valid logical value (equivalent to iter().flatten()), and it's the wholesale append that drives the speedup. retract_batch's null-free branch mirrors it.
• retract error: checking !to_remove.is_empty() after the removal loop is the right adaptation for the non-distinct multiset — any leftover entries are values that were retracted but never present, which is exactly the divergence worth surfacing rather than silently dropping. Consistent with the internal_err! we added on the distinct side.

Test coverage is good: the error-path test and the dense-vs-sparse update_batch equivalence test cover both new branches. LGTM.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.47312% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.75%. Comparing base (b0b9dae) to head (886dbef).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...afusion/functions-aggregate/src/percentile_cont.rs	89.47%	4 Missing ⚠️
datafusion/functions-aggregate/src/median.rs	94.54%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23954      +/-   ##
==========================================
+ Coverage   80.69%   80.75%   +0.05%     
==========================================
  Files        1095     1096       +1     
  Lines      372529   373511     +982     
  Branches   372529   373511     +982     
==========================================
+ Hits       300626   301627    +1001     
+ Misses      53942    53895      -47     
- Partials    17961    17989      +28     

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